A. Field of the Invention
This invention relates to the field of hadron beams in synchrotrons designed for acceleration. More particularly, the present invention relates to a method for decelerating hadron beams using existing synchrotrons designed for acceleration. Even more specifically, the present invention relates to a method for decelerating antiprotons using existing synchrotrons designed for acceleration. More specifically, the present invention addresses the production of antiprotons; the collection and storage of antiprotons; the transport of antiprotons.
B. Background of the Invention
Hadron beams are typically accelerated using synchrotrons, cyclotrons, or linear accelerators. For example, at the Loma Linda proton therapy facility a synchrotron is employed once the protons are emitted by the ion source and pre-accelerated in a radio-frequency quadrupole (RFQ), while the Massachussetts General Hospital proton therapy facility employs a cyclotron. They accelerate protons up to a momentum of 0.73 GeV/c, which corresponds to the energy a proton needs to completely traverse a typical human chest cavity.
There are examples of synchrotrons specifically designed for deceleration of hadron beams. These include the LEAR and AD synchrotrons, both operated at the CERN particle physics laboratory in Geneva, Switzerland. These synchrotrons are used to perform scientic experiments with antiprotons.
A third category of synchrotrons, called storage rings, neither accelerate nor decelerate hadron beams to higher or lower momenta. Their purpose is to merely store the hadron beam at their original injection momentum.
Because the radius of curvature of a hadron beam traversing bending magnets is proportional to the beam momentum, and the cost of the synchrotron scales with its circumference, the synchrotron is designed such that the bending (dipole) magnets are at their maximum field strength at the maximum anticipated momentum. Therefore, storage rings always operate at the maximum strength of their magnets, while accelerating synchrotrons operated at maximum magnetic field strength only at the end of the acceleration process.
All mechanical and electrical systems have a finite dynamic range within which the components can operate. The same is true of synchrotrons. Due to limitations in magnetic material properties, power supply regulation, and radio frequency acceleration system frequency adjustability, synchrotrons traditionally are found to have a maximum momentum range of a factor of twenty. There is a great deal of literature devoted to this issue in the field of accelerator physics. This reality also explains why laboratories working in the fields of atomic, nuclear, and particle physics have accelerator chains composed of many synchrotrons. The Fermi National Accelerator Laboratory is an example, wherein there are three synchrotrons required to accelerate protons and antiprotons to a momentum of 950 GeV/c for particle physics research. The maximum momentum range of any of these synchrotrons is a factor of 17.
One aspect of the present invention has an object of providing a means to better decelerate hadron beams. The corresponding method for decelerating antiprotons opens many commercial applications. For example, antiproton irradiation has utility in a variety of fields, including the treatment of cancerous tissue and the generation of radioisotopes within the body that are useful for imaging techniques and therapeutic treatment. In the present invention deceleration is implemented using a synchrotron.
A synchrotron is comprised of a ring of dipole magnets interspersed with quadrupole, sextupole, correction dipole magnets, and one or more radio frequency acceleration systems, the operation of which are all managed by a computer control system. The dipole magnets bend a hadron beam into a closed loop that repeatedly passes through the electromagnetic fields generated by the radio frequency acceleration system. Typically all dipole magnets are wired in series to ensure that every magnet has exactly the same electrical current, and therefore magnetic field strength, in it. One or more power supplies around the synchrotron are employed to provide this electrical current. The amount of electrical current generated by a power supply at any given momentum is determined by commands from the computer control system.
The quadrupole magnets, which focus and defocus the hadron beam in a fashion very similar to the concave and convex lens combination in a telephoto camera lens, make sure that the beam oscillates around the middle of the magnets rather than straying out of the synchrotron. The strength of the focusing and defocusing magnetic fields are adjusted to maintain a desired number of horizontal and vertical oscillations each turn (revolution) around the synchrotron. Typically all of the focusing quadrupoles are wired in series to ensure that every magnet has exactly the same electrical current and hence magnetic field strength. Similarly, all of the defocusing quadrupoles are also wired in series. Two or more power supplies around the synchrotron are employed to provide these two electrical currents. The amount of electrical current generated by a power supply at any given momentum is determined by commands from the computer control system.
The sextupole magnets are used to control the horizontal and vertical chromacities of the synchrotron. Every hadron beam has some non-zero momentum distribution width. Without sextupoles, every hadron with a different momentum would have a different number of horizontal and vertical oscillations per revolution, or horizontal and vertical tunes. The change in tune per unit change in momentum is called chromaticity. The natural chromaticity of a synchrotron without sextupoles is equal and opposite to the tune. But too keep the hadron beam in the synchrotron for the desired duration, it is necessary to impose sextupole magnetic fields that simultaneously reduce both the horizontal and vertical chromaticity to near zero. Typically there is one sextupole placed near every quadrupole, with sextupoles near focusing quadrupoles having one field and the sextupoles near defocusing quadrupoles having a nearly equal but opposite field. Typically, all of the focusing xe2x80x9cfocusingxe2x80x9d sextupoles are wired in series to ensure that every magnet has exactly the same electrical current and hence magnetic field strength. Similarly, all of the xe2x80x9cdefocusingxe2x80x9d sextupoles are also wired in series. Two or more power supplies around the synchrotron are employed to provide these two electrical currents. The amount of electrical current generated by a power supply at any given momentum is determined by commands from the computer control system.
The position and orientation of each magnet always has some tolerance of misalignment. In addition, the strength of every dipole magnet is not precisely equal. These accumulated errors cause the hadron beam to deviate away from the magnet centers. Dipole correction magnets are used to steer a hadron beam vertically and horizontally, correcting the overall beam trajectory. There is typically one horizontal dipole corrector magnet at each focusing quadrupole and one vertical dipole corrector magnet at each defocusing quadrupole. Because the distribution of errors is typically random and time variable, each dipole corrector magnet has a unique electrical current generated by a separate power supply. The amount of electrical current generated by a power supply at any given momentum is determined by commands from the computer control system.
In a linear accelerator, each hadron in the beam passes once through each radio frequency acceleration cavity supporting electromagnetic fields. Just as a surfer rides the edge of a wave to pick up speed, each charged hadron is accelerated or decelerated by riding either the leading or trailing edge of the electromagnetic waves. Each cavity and the multiple radio frequency amplifiers that power them are some of the most expensive elements of any particle accelerator. The innovation behind synchrotrons is that the hadron beam is looped around to reuse the same cavities multiple times, receiving momentum changes tens or hundreds of thousands of time per second. In this way, larger overall momentum changes are implemented at a fraction of the cost if implemented using a linear accelerator.
The importance of this invention, the modifying of a synchrotron designed for hadron beam acceleration in order to decelerate hadron beams, is the immense savings in time, manpower, and money over designing and building a dedicated synchrotron for deceleration. Whereas the construction of a new synchrotron can cost anywhere between $10 million and $1 billion, depending on the maximum momentum required, this modification of an existing synchrotron can cost as little as $10,000.
These and other features, objectives and advantages of the present improved invention will be readily understood upon consideration of the following detailed description of certain embodiments of the present improved invention and the accompanying drawings.
However, as a summary overview, the present invention provides a method for modifying an existing synchrotron designed for the acceleration of hadron beams to higher momenta, such that the synchrotron is enabled to decelerate hadron beams instead to lower momenta. These modifications can be made, for example, to certain synchrotron equipment and computer control system hardware and software to produce a counterintuitive use of a synchrotron designed for accelerating, i.e., decelerating.
Currently, antiprotons are generated and used in experimental studies of elementary particles physics. These experiments are typically performed at large particle accelerators, such as the Tevatron at the Fermi National Accelerator Laboratory (Fermilab). The Fermilab accelerator complex includes various linear accelerators and synchrotrons that are designed to generate antiprotons, to accelerate these antiprotons to very high energies and momenta (typically to 1 TeV), and to collide these antiprotons together with protons. The results of the collisions can be analyzed to provide information regarding the structure and physical laws of the universe.
While these experimental studies of particle physics use antiprotons with very high energies and momenta, other uses of antiprotons, such as the medical use, have relatively small energies and momenta. If the existing sources of antiprotons at such accelerators are to be used as sources of antiprotons for these other fields, the antiprotons have to be decelerated (i.e., energy and momentum of the antiprotons will have to be reduced). In addition, to provide antiprotons to locations that are off-site from the particle accelerators, the antiprotons have to be decelerated sufficiently to enable them to be stored in a portable synchrotron or cyclotron, or trapped in a container and transported to other locations. Because antiprotons are annihilated upon contacting matter, development has been performed to develop adequate containers (e.g., Penning traps) for transporting antiprotons. Further details regarding such methods are incorporated by reference, including: xe2x80x9cContainer for Transporting Antiprotons,xe2x80x9d U.S. Pat. No. 5,977,554 issued to Gerald A. Smith, et al. on Nov. 2, 1999; xe2x80x9cContainer for Transporting Antiprotons,xe2x80x9d U.S. Pat. No. 6,160,263 issued to Gerald A. Smith, et al. on Dec. 12, 2000.
Embodiments of the present invention decelerate the antiprotons by operating existing particle accelerators, which were designed to accelerate the antiprotons, under conditions that actually reduce the energy and momentum of the antiprotons. In sum, though, there is a method of decelerating antiprotons, the method comprising the steps of: providing antiprotons to a particle accelerator ring, the antiprotons having a first momentum distribution with a first average momentum; operating the particle accelerator ring so as to apply electromagnetic fields to the antiprotons as the antiprotons travel around the ring; and selectively applying the electromagnetic fields to the antiprotons as the antiprotons travel around the ring, such that the antiprotons have a second momentum distribution with a second average momentum less than the first average momentum. Another embodiment of this same idea can be phrased as a method for decelerating antiprotons includes providing antiprotons to a particle accelerator ring. The antiprotons have a first momentum distribution with a first average momentum. The method further includes operating the particle accelerator ring so as to apply electromagnetic fields to the antiprotons as the antiprotons travel around the ring. The method further includes selectively applying the electromagnetic fields to the antiprotons as the antiprotons travel around the ring such that the antiprotons have a second momentum distribution with a second average momentum less than the first average momentum.